Generally, IC cards used as mobile electronic devices comprise a main body shaped like a card and made of plastic, and an IC module embedded in the main body. The IC module has an IC chip. The IC chip has a nonvolatile memory and a CPU. The nonvolatile memory is, for example, an electrically erasable programmable read-only memory (EEPROM) or a flash read-only memory (ROM), which can hold data even if not supplied with power. The CPU performs various arithmetic operations.
The IC card excels in portability and can perform communication with terminals and complex arithmetic operations. Further, the IC card, which is hard to forge, is intended to store highly classified information, and to be used in security systems and electronic transactions.
In recent years, IC cards that can transmit and receive data in non-contact communication have come into general use. All contactless IC cards have an IC chip and an antenna. The contactless IC card receives a magnetic field emanating from the reader/writer provided on a terminal device that processes data stored in the IC card. The antenna provided in the card operates via electromagnetic induction.
A plurality of IC cards may exist in an area within which the IC cards can communicate with one another through a terminal device. In this case, the terminal device may fail to identify the IC cards correctly. In order to prevent this state known as “collision”, thereby to identify the IC cards correctly, the terminal device can perform an anti-collision process in the slot marker system.
In the slot marker system, the terminal device transmits an initial response command, which contains the data representing the total number of slots, to the plurality of contactless IC cards. In each contactless IC card, a logic circuit generates n (integer) ranging from 0 to (N−1). If n=0, the contactless IC card immediately makes an initial response to the terminal device. If n is not 0, the contactless IC card makes no response immediately, and transmits the initial response to the terminal device when it receives from the terminal device a slot marker command that designates the slot identical to n generated by the contactless IC card.
The order in which to perform processes may be predetermined in the contactless IC card. For example, two contactless IC cards, both issued by a financial institute, may be used to settle a transaction. More precisely, the main contactless IC card may be first used to settle a specific amount, and the sub contactless IC card may then be used to settle the remaining amount.
The slot marker system based on ISO:IEC14443 Type B, however, cannot guarantee that the main contactless IC card can always make the initial response before the sub contactless IC card. This is because the main card and the sub card generate n slots, independently of each other.